Mechanical stabilization of lattice mismatched quantum wells

ABSTRACT

In order to achieve a long wavelength, 1.3 micron or above, VCSEL or other semiconductor laser, layers of strained quantum well material are supported by mechanical stabilizers which are nearly lattice matched with the GaAs substrate, or lattice mismatched in the opposite direction from the quantum well material; to allow the use of ordinary deposition materials and procedures. By interspersing thin, unstrained layers of e.g. gallium arsenide in the quantum well between the strained layers of e.g. InGaAs, the GaAs layers act as mechanical stabilizers keeping the InGaAs layers thin enough to prevent lattice relaxation of the InGaAs quantum well material. Through selection of the thickness and width of the mechanical stabilizers and strained quantum well layers in the quantum well, 1.3 micron and above wavelength lasing is achieved with use of high efficiency AlGaAs mirrors and standard gallium arsenide substrates.

Application Priority

This invention is being filed as a continuation application and claimspriority to nonprovisional application Ser. No. 09/217,223, filed Dec.21, 1998 now U.S. Pat. No. 6,603,784 by Ralph H. Johnson, entitled“Mechanical Stabilization of Lattice mismatched Quantum Wells.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to vertical cavity surface emitting lasers. Theinvention relates specifically to longer wavelength VCSELs such as 1.3micrometer, or micron, (μm) wavelengths which can be made with ordinaryMOCVD equipment or MBE equipment. In general it relates to obtaininglight emission at wavelengths not normally obtainable with a givenmaterial system because of lattice mismatch.

2. Description of the Related Art

Vertical cavity surface emitting lasers (VSCEL) made with GaAs are knownin the art which emit light in the 850 nanometer range. Because thequantum well for the short wavelength 850 nanometer VCSELs is made fromGaAs (the same material as the substrate) the various epitaxiallydeposited layers, whose thickness is related to wavelength, are able tomaintain the minimal mechanical strain without mechanical relaxation.However, if one were to use InGaAs in the active region at the larger1.3 micron wavelengths, the lattice mismatch is so large the layerswould tend to relax their strains and suffer dislocations, slip lines orisland growth which would interfere with proper lasing.

In order to go to the proper bandgap for a 1.3 μm wavelength VCSEL onemust use InGaAs or GaAsSb or some combination thereof instead of GaAs inthe active layer. However, indiumgalliumarsenide andgalliumaresenideantimonide are not the same lattice constant as GaAs atthe compositions useful for 1.3 micron lasers. This makes it verydifficult to build a proper quantum well structure.

It is therefore very desirable to come up with a quantum well (i.e. theactive layer and the barrier layers surrounding it) which makes use ofcommon GaAs, InGaAs or GaAsSb materials in construction of the 1.3micron wavelength VCSEL.

SUMMARY OF THE INVENTION

The present invention extends the use of nonlattice matched quantumwells by extending the composition range over which they aremechanically stable. This is done by introducing thin regions, ormechanical stabilizers in the quantum well region, with the same latticeconstant as the substrate while using thin layers of a semiconductoralloy of a different lattice constant in the quantum well structure.Alternatively, the lattice constant of the mechanical stabilizers may benearly, e.g. about ±2%, the same as that of the substrate, or mismatchedin the opposite direction of the remainder of the quantum well material.The mechanical stabilizers are thin enough that their effect on thequantum well energy levels is small enough to be convenientlycompensated for by modifying the composition, i.e. the indium to galliumratio of the InGaAs layers or the arsenic to antimony ratio of GaAsSb ora combination of the above in InGaAsSb. A series of mechanicalstabilizers is created within the quantum well structure. The effectivequantum well energy level is that from the whole series of quantum wellstructures with mechanical stabilization layers therein. The effectivequantum well energy level is modified only slightly by the presence ofthe mechanical stabilizers.

The purpose of this amendment is to correct a typographical error. Nonew matter is believed to be entered as a result of this amendment tothe specification.

The mechanical stability is guaranteed by keeping the strained quantumwell material between the stabilizers about or below the criticalthickness as defined by Matthews and Blakeslee for nonlattice matchedcrystal growth. See for example p. 374 of ‘Quantum Well Lasers,’ PeterZory, Academic Press 1993 for an interpretation of different criticalthickness models including Matthews and Blakeslee. The mechanicalstabilizers are unstrained since they are the same lattice constant asthe substrate. The present invention may be generally used, butspecifically applies to GaAs substrates; InGaAs, GaAsSb, or InGaAsSbquantum wells and GaAs mechanical stabilizers, or combinations thereof.

With the use of the mechanical stabilizers of the present inventionactive layer structures of the VCSEL may be built from common InGaAs orGaAsSb and GaAs materials used with ordinary MOCVD deposition equipmentat layer thicknesses suitable for 1.3 micron wavelength emission withoutrelaxation of mechanical strain; leading to reliable lasing in thiswavelength with the use of common deposition methods and materials.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully and completely understood froma reading of the Description of the Preferred Embodiment in conjunctionwith the drawings, in which:

FIG. 1 is a schematic representation of a VCSEL according to the presentinvention.

FIG. 2 is a schematic illustration of InGaAs lattice relaxation on aGaAs substrate.

FIG. 3 is a schematic view of the energy bands versus depth of an activeportion of a 1.3 micron VCSEL according to the present invention.

FIG. 4 is a schematic view of an alternative quantum well structureaccording to the present invention.

FIG. 5 is a schematic representation of the mechanical energy within themechanically stabilized InGaAs quantum well using the GaAs stabilizationlayers.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Throughout the Description of the Preferred Embodiment, like componentswill be identified by like reference numerals.

As seen in FIG. 1, a VCSEL 11 has, as viewed from the bottom up, a metalcontact layer 13 adjacent and a first conductivity type, in this case Ntype, substrate 15 upon which is deposited an N type mirror stack 17.The active region 19 is adjacent the N type mirror stack and iscomprised of GaAs barrier layers and InGaAs quantum well layer asfurther explained below. On top of the active layer, 19 is deposited asecond conductivity type, in this case P type, mirror stack 21 uponwhich is deposited a P metal contact layer 23. A current blocking region24, as known in the art is disposed in the P type mirror stack 21.

Although structures detailed in the preferred embodiment, except theactive layer, are of ordinary construction; other structures or layersnot detailed herein but known to those having ordinary skill in the artmay of course be added to the structures presented herein.

As discussed above, there are certain problems with maintainingmechanical stress in long wavelength VCSEL layers necessary for 1.3micron emission; when attempting to use GaAs substrates with InGaAsquantum well layers, and AlGaAs mirrors, i.e. common materials depositedthrough the use of common MOCVD equipment.

As seen in FIG. 2, a schematic representation of a GaAs layer 25 uponwhich is deposited an InGaAs layer 27, because these two materials havedifferent lattice constants, when one attempts to deposit too thick of alayer of InGaAs upon the GaAs layer beneath it, or substrate, at acertain point the mechanical strain of the InGaAs will relax, as at 29,causing a dislocation, slip line, or damage point which will negate orinterfere with proper lasing activity. Unfortunately, a certainthickness must be maintained in order to obtain the proper energy levelsto produce the longer wavelength lasing, i.e. 1.3 micron. Thus theInGaAs layers must be made thinner.

As shown in FIG. 3, an energy versus position plot, a two hundred twentyfive angstrom quantum well 33 is composed of InGaAs and surrounded oneither side by barrier layers 31 composed of GaAs. Within the quantumwell structure 33 are located six substantially equidistant, 9.5 Åthick, gallium arsenide spacer layers 37 surrounded by seven InGaAslayers 39 of approximately 24 Å thickness. A wavefunction line 30 andminimum allowable energy line 20 for the active region 19 are includedin the plot. There may be other arrangements of GaAs spacer layers, suchas two or four layers within the quantum wells, and it is probable thatthe InGaAs and GaAs layer widths will have to be multiples of thelattice constant. Thus the thickness of the quantum well may changeslightly to achieve optimal lasing performance.

It will be noted that the mechanically stabilized quantum wave functionsextend into the GaAs barrier layers 31. The dimensions are selected suchthat the lattice strain of the mechanically reinforced InGaAs layers 39causes band splitting that modifies the InGaAs bandgap. The GaAsmechanical stabilizer layer thicknesses, the InGaAs layer thicknesses,the InGaAs composition and the total well thickness or width, willdetermine the position of the quantum levels relative to the band edge.However, it is believed that the dimensions shown are closeapproximations to desirable for indium 0.7 gallium 3 arsenidecomposition of the InGaAs layer.

As shown in FIG. 4, alternative forms of a quantum well may beconstructed according to the present invention. The quantum well 35 maybe about two hundred angstroms wide with a superlattice of equidistantstabilization layers of 11.2 angstrom GaAs substrate material surroundedby InAs semiconductor alloy layers 49 of each about 12 angstroms.

The mechanical stabilization layered quantum wells according to thepresent invention are to be construed using ordinarily known etching anddeposition techniques for standard MOCVD equipment.

The quantum wells of the present invention are surrounded by GaAsbarrier layers upon which it is suitable to deposit high efficiencyAlGaAs mirrors whose lattice constant matches that of the GaAs barrierlayers. A mechanical energy graph representation line 41 is shown inFIG. 5 to illustrate that the strain is kept on the InGaAs layer at alevel above that of the GaAs mechanical stabilizers 37 which is in anunstrained state due to lattice constant matching. During the growthprocess the strained epitaxial layer follows the lattice constant of thesubstrate until it passes the critical thickness. At this thicknessinstead of maintaining the strain it is relaxed with dislocations. Bykeeping the thickness under the critical thickness the layers do notrelax and form dislocations. The GaAs mechanical stabilizers are notstrained because they follow the lattice constant of the substrate.Growing the following InGaAs layer on the GaAs mechanical stabilizer isidentical to growing it on the substrate.

Thus by following the teachings of the present invention a 1.3 micronwavelength VCSEL may be manufactured utilizing quantum wells of InGaAs,or other semiconductor compounds, with gallium arsenide mechanicalstabilization layers in order to keep the semiconductor layers thinenough to maintain mechanical strain while utilizing common AlGaAsmirror structures.

1. A semiconductor laser comprising: an active layer that includes atleast one quantum well, the at least one quantum well includingsemiconductor alloy layers under mechanical stress and stabilizingmaterial layers, wherein the stabilizing material layers areinterspersed between the semiconductor alloy layers and serve asmechanical stabilizers for the semiconductor alloy layers; barrierlayers sandwiching the active layer; and mirror layers disposed outsideof the barrier layers.
 2. The invention of claim 1, wherein thesemiconductor laser is one of a vertical cavity surface emitting laser,an edge emitting laser, or a light emitting diode.
 3. The invention ofclaim 1, wherein the semiconductor alloy layers are comprised of one ofInGaAs, GaAsSb, or InGaAsSb, and the substrate is comprised of GaAs. 4.The invention of claim 3, wherein the mirror layers are comprised ofAlGaAs.
 5. The invention of claim 4, wherein the quantum wells are about80 Å–250 Å thick.
 6. The invention of claim 4, wherein the quantum wellmechanical stabilizer layers are about 9.5 Å–11.2 Å thick.
 7. Theinvention of claim 4, wherein the alloy layers are about 24 Å thick. 8.The invention of claim 1, wherein the stabilizing layers aresubstantially unstrained.
 9. A semiconductor laser, comprising: asubstrate; and an active layer disposed above the substrate andcomprising: at least one quantum well that includes within it:semiconductor alloy layers under mechanical stress; and stabilizingmaterial layers that are arranged in alternating fashion with thesemiconductor alloy layers, the stabilizing layers serving as mechanicalstabilizers for the semiconductor alloy layers; barrier layerssandwiching the active layer; and mirror layers disposed outside of thebarrier layers.
 10. The semiconductor laser as recited in claim 9,wherein the stabilizing layers are substantially lattice matched withthe substrate.
 11. The semiconductor laser as recited in claim 9,wherein the semiconductor laser comprises one of: a vertical cavitysurface emitting laser; an edge emitting laser; or a light emittingdiode.
 12. The semiconductor laser as recited in claim 9, wherein thesemiconductor alloy layers are comprised of one of InGaAs, GaAsSb, orInGaAsSb, and the substrate is comprised of GaAs.
 13. The semiconductorlaser as recited in claim 9, wherein the stabilizing layers aresubstantially unstrained.